Light olefin products (e.g. ethylene, propylene, and butene) generated by various technologies such as gas to olefins, methanol to olefins, steam cracking or fluid catalytic cracking contain highly unsaturated impurities, namely, alkynes and alkadienes, as by-products. These impurities must be removed from light olefins because they are poisons to downstream olefin polymerization catalysts. Currently, selective hydrogenation of alkynes and alkadienes, such as acetylene, methyl acetylene (MA), propadiene (PD), and/or butadiene (BD), into the respective olefins is the most attractive technology option for olefin manufacturing plants. Traditionally, catalysts such as nickel or palladium supported on alumina have been used for the selective hydrogenation. Palladium-based catalysts, however, are becoming the workhorse of the industry by gradually replacing the older nickel-based catalysts.
The selective hydrogenation of acetylene and/or MAPD and/or BD is typically carried out in four unit types:
Front-End Selective Catalytic Hydrogenation Reactors, where the feed is composed of C3 and lighter hydrocarbons, or C2 and lighter hydrocarbons. In the case of raw gas applications, other components such as butadiene, ethyl acetylene, dimethyl acetylene, vinyl acetylene, cyclopentadiene, benzene, and toluene can also be present.
Back-End Selective Catalytic Hydrogenation Reactors, where the feed is composed of an ethylene-rich stream.
MAPD Selective Catalytic Hydrogenation Reactors, where the feed is composed of a propylene-rich stream.
BD Selective Catalytic Hydrogenation Reactors, where the feed is composed of a butylene-rich stream.
Current commercial selective hydrogenation catalysts suffer from the problems of producing significant amounts of saturates (e.g. ethane, propane, butane) and green oil (C4+ oligomer compounds). The saturates come from over-hydrogenation of the alkynes and/or alkadienes and the hydrogenation of olefins. Green oil is the result of oligomerization of the alkynes and/or alkadienes and/or olefins. Both saturates and green-oil are undesirable owing to their adverse effect on olefins-gain selectivity. Green oil, however, is especially troublesome in that it also decreases catalyst life by depositing heavy compounds on catalyst surfaces.
It would be desirable to have catalysts and a process for the accurate and controlled hydrogenation of alkadienes and alkynes in olefin product streams for both economic and operational benefits including, but not necessarily limited to, provision of more consistent product quality, reduction in the amount of olefin hydrogenated to ethane in the acetylene reactor, elimination of olefin production loss due to acetylene reactor shut-down required by process upsets, extension of the life of catalysts by elimination of reactor runaways, and increase in run time between regeneration of catalyst by reduced formation of heavy hydrocarbon poisons, and reduction of overall hydrogen consumption.